Parameterized constructor in Java - Mini Project: Build & Apply

Parameterized constructor

📖 Scenario: You are creating a simple Java program to represent a Book in a library system. Each Book has a title and an author.

🎯 Goal: Build a Java class Book with a parameterized constructor to set the title and author when creating a new Book object.

📋 What You'll Learn

Create a class called Book with two String fields: title and author.

Add a parameterized constructor to Book that takes two parameters: title and author.

Create a main method in a class called Library.

In main, create a Book object using the parameterized constructor with the title "The Alchemist" and author "Paulo Coelho".

Print the book's title and author using System.out.println.

💡 Why This Matters

🌍 Real World

Parameterized constructors are used in real-world programs to create objects with specific starting values, like setting up a book's title and author when adding it to a library system.

💼 Career

Understanding constructors is essential for Java programming jobs, as it helps in creating flexible and reusable code for object initialization.

1

Create the Book class with fields

Create a class called Book with two String fields named title and author.

Java

public class Book {
    // Your code here
}

Hint

Think of title and author as labels on a book to store its name and writer.

2

Add a parameterized constructor

Add a parameterized constructor to the Book class that takes two parameters: String title and String author, and assigns them to the class fields.

Java

public class Book {
    String title;
    String author;

    // Add constructor here
}

Hint

The constructor sets the book's title and author when you create a new Book object.

3

Create the Library class with main method

Create a class called Library with a main method. Inside main, create a Book object named myBook using the parameterized constructor with title "The Alchemist" and author "Paulo Coelho".

Java

public class Book {
    String title;
    String author;

    public Book(String title, String author) {
        this.title = title;
        this.author = author;
    }
}

public class Library {
    public static void main(String[] args) {
        // Create Book object here
    }
}

Hint

The main method is the starting point of the program where you create your book.

4

Print the book's title and author

In the main method of Library, add two System.out.println statements to print the title and author of myBook.

Java

public class Book {
    String title;
    String author;

    public Book(String title, String author) {
        this.title = title;
        this.author = author;
    }
}

public class Library {
    public static void main(String[] args) {
        Book myBook = new Book("The Alchemist", "Paulo Coelho");
        // Print title and author here
    }
}

Hint

Use System.out.println(myBook.title); and System.out.println(myBook.author); to show the book details.

Practice

1.

What is the main purpose of a parameterized constructor in Java?

easy

A. To initialize an object with specific values when it is created

B. To create multiple objects without any initial values

C. To delete an object from memory

D. To define a method that returns a value

Solution

1. Step 1: Understand constructor purpose

   A constructor is used to initialize objects when they are created.

2. Step 2: Identify parameterized constructor role

   A parameterized constructor takes arguments to set initial values for the object's fields.

3. Final Answer:

   To initialize an object with specific values when it is created -> Option A

4. Quick Check:

   Parameterized constructor = initialize with values [OK]

Hint: Constructors with parameters set initial object values [OK]

Common Mistakes:

• Confusing constructors with methods
• Thinking constructors delete objects
• Believing constructors don't take parameters

2.

Which of the following is the correct syntax for a parameterized constructor in Java?

public class Car {
    String model;
    int year;

    // Constructor here
}

easy

A. void Car(String model, int year) { this.model = model; this.year = year; }

B. public Car(String model, int year) { this.model = model; this.year = year; }

C. public Car() { model = ""; year = 0; }

D. public void Car(String model, int year) { model = model; year = year; }

Solution

1. Step 1: Check constructor syntax

   A constructor has no return type and matches the class name exactly.

2. Step 2: Verify parameter usage

   public Car(String model, int year) { this.model = model; this.year = year; } correctly uses parameters and assigns them to fields with this.

3. Final Answer:

   public Car(String model, int year) { this.model = model; this.year = year; } -> Option B

4. Quick Check:

   Constructor syntax = no return type + class name [OK]

Hint: Constructor name = class name, no return type [OK]

Common Mistakes:

• Adding return type to constructor
• Not using 'this' to assign fields
• Using void or other return types

3.

What will be the output of the following Java code?

class Book {
    String title;
    int pages;

    Book(String title, int pages) {
        this.title = title;
        this.pages = pages;
    }

    void display() {
        System.out.println(title + ": " + pages + " pages");
    }
}

public class Main {
    public static void main(String[] args) {
        Book b = new Book("Java Basics", 250);
        b.display();
    }
}

medium

A. Java Basics: 250 pages

B. Java Basics pages

C. 250 pages

D. Compilation error

Solution

1. Step 1: Understand constructor initialization

   The constructor sets title to "Java Basics" and pages to 250.

2. Step 2: Analyze display method output

   The display() method prints the title, colon, pages, and "pages" text.

3. Final Answer:

   Java Basics: 250 pages -> Option A

4. Quick Check:

   Constructor sets fields, display prints them [OK]

Hint: Constructor sets fields, display prints combined string [OK]

Common Mistakes:

• Ignoring 'this' keyword effect
• Expecting only title or pages printed
• Assuming syntax error without reason

4.

Identify the error in this Java class with a parameterized constructor:

public class Student {
    String name;
    int age;

    public Student(String n, int a) {
        name = n;
        age = a;
    }

    public Student() {
        name = "Unknown";
        age = 0;
    }

    public void Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

medium

A. No default constructor defined

B. Missing return type in parameterized constructor

C. The method named Student with void return type is not a constructor

D. Fields name and age are not declared

Solution

1. Step 1: Check constructor definitions

   Constructors have no return type and match class name exactly.

2. Step 2: Identify method named Student with void

   The method public void Student(String name, int age) is not a constructor but a method, which is confusing.

3. Final Answer:

   The method named Student with void return type is not a constructor -> Option C

4. Quick Check:

   Constructors have no return type [OK]

Hint: Constructors never have a return type, not even void [OK]

Common Mistakes:

• Writing constructors with void return type
• Confusing methods with constructors
• Missing default constructor when needed

5.

Given the class below, what will be the output when creating two Employee objects with different parameters and printing their details?

class Employee {
    String name;
    double salary;

    public Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    public void printInfo() {
        System.out.println(name + " earns $" + salary);
    }
}

public class Test {
    public static void main(String[] args) {
        Employee e1 = new Employee("Alice", 50000);
        Employee e2 = new Employee("Bob", 60000);
        e1.printInfo();
        e2.printInfo();
    }
}

hard

A. Runtime error due to uninitialized fields

B. Alice earns $60000.0
Bob earns $50000.0

C. Compilation error due to missing default constructor

D. Alice earns $50000.0
Bob earns $60000.0

Solution

1. Step 1: Analyze object creation with parameters

   Two Employee objects are created with names and salaries passed to the parameterized constructor.

2. Step 2: Check printInfo output

   Each object's printInfo() prints the name and salary correctly.

3. Final Answer:

   Alice earns $50000.0 Bob earns $60000.0 -> Option D

4. Quick Check:

   Parameterized constructor sets fields, printInfo shows them [OK]

Hint: Each object holds its own values set by constructor [OK]

Common Mistakes:

• Mixing up values between objects
• Expecting errors without reason
• Assuming default constructor is needed here